128 MAZATLAN BIVALVES 
intermediate in texture between the shell and the calcareous 
incrustation. This deposit extends about half way down the 
burrow. This species and L. plumula are always found close 
to the outside of the matrix; but they do not affect the same 
situations. In Patella Mexicana, L. aristatus is often very com- 
mon. In about one fifth of an aged limpet Mr. Darbishire found 
13 large specimens, without reckoning empty burrows. Here 
however, and in Chama, L. plumula is extremely rare; while 
in the thick lower valves of Spondylus it is not uncommon. 
The burrows are shaped nearly as the shell, with no power of 
rotatory motion, and very little scope for opening the valves. 
Traces of the foot mark are very rare. The shell, when adult, 
is more or less incurved ventrally, and angled dorsally ; tumid 
at both extremities. Shell (without epidermis and incrustation) 
extremely thin. The largest specimen found perfect measures 
long. (with the beaks) 1°56, lat. ‘47, alt. *4. 
Hab.—Senegal and West Indies; found in ballast, London 
roads (!), Horbes.—Senegal, in shells of Balani, Adanson.— 
Abundant in Ostrea iridescens, W. coast Africa, Stutchbury : 
—do. do. Anamaboa, B. M.—Guinea, Yams.—Red Sea, 
Dunker.—St. Thomas, Hornbeck.—Mazatlan ; abundant in 
Cham, Spondyli, Ostrea iridescens, Patella Mexicana and 
discors, Imperator unguis and olivaceus, Strombus galea, 
&e; L’pool & Havre Coll. 
Tablet 581 contains 11 pairs, and 8 pairs of valves, extremely 
young.—582, 5 pairs and 4 pairs of valves, a stage older.—583, 
8 pairs and 3 pairs of valves, do.—584, 6 sp. adolescent.—5835, 
5 do. adult. 
Tablet 586 contains 6 young sp. in situ, burrowing in Patella 
Mexicana.—587, 1 do. do. with the beaks detached, shewing the 
glossy epidermis beneath.—588, 2 adult sp. in P. Mexicana.— 
589, fragment of P. Mexicana, displaying shelly lining to 
tubes.—590, another fragment, in which a Lithophagus had 
bored ‘6 beyond the inner surface ; the limpet having defended 
itself from its pursuer by fresh layers of shell—591, 4 sp. of 
Patella discors, variously distorted by Lithophagi.—592, Fis- 
surella rugosa and F. alba, similarly attached.—593, Mytilus 
tenuiaratus, with 3 Lith. in situ: part of the mussel being 
broken away displays its jasper-like texture, as well as the 
shelly lining of the tube.—594, 3 sp. of Imperator unguis, with 
Lith. in situ; in one, they have just reached the interior of 
the mouth; another is bored across the apex; the other, not 
finding room within, has increased its size by raising a tumulus 
